1. Technical Field
The present disclosure relates generally to electrical relays, and more particularly, but not necessarily entirely, relays that switch at specified instances.
2. Background Art
Relays are used as switches to control power to electrical devices. A relay may be defined as an electromechanical switch operated by a flow of electricity in one circuit and controlling the flow of electricity in another circuit. A relay may consist basically of an electromagnet with a soft iron bar, called an armature, held close to it. A movable contact is connected to the armature in such a way that the contact is held in its normal position by a spring. When the electromagnet is energized, it exerts a force on the armature that overcomes the pull of the spring and moves the contact so as to either complete or break a circuit. When the electromagnet is de-energized, the contact returns to its original position. Variations on this mechanism are possible: some relays have multiple contacts; some are encapsulated; some have built-in circuits that delay contact closure after actuation; some, as in early telephone circuits, advance through a series of positions step by step as they are energized and de-energized.
Since the actuation of a relay requires the physical movement of one of the contact electrodes, there may be some delay from the issuance of a close command until the magnetic field has build to a sufficient level to begin movement of the contact electrodes by overcoming the spring force. This delay makes it difficult to precisely time the actual opening or closing of the electrodes.
Relays are often used to switch alternating current (AC). AC occurs when charge carriers in a conductor or semiconductor periodically reverse their direction of movement. Household utility current in the U.S. and some other countries is AC with a frequency of 60 hertz (60 complete cycles per second), although in other countries it is 50 Hz.
An AC waveform may be sinusoidal, square, or sawtooth-shaped. Some AC waveforms are irregular or complicated. An example of sine-wave AC is common household utility current (in the ideal case). One characteristic of the AC waveform is that it crosses zero when reversing directions. At this zero crossing point, there is no current flowing.
The voltage of an AC power source also changes from instant to instant in time. The AC voltage changes is also a sinusoidal wave that over time starts at zero, increases to a maximum value, then decreases to a minimum value, and repeats.
In applications where relays are repeatedly switched, the life of the relay may be cut short by arcs (a luminous bridge of ionized gas) that form across the relay contacts when switched. The time period in which the arc flows is determined by many factors including the mechanical bounce of the contracts upon closure, the distance between the contact electrodes, the magnitude of the current flowing, as well as the level of ionization of the air in the gap between contact electrodes.
These arcs may cause pits and welds to accumulate on the contact surface which diminish the useful life of the relay. The pits are formed through a small portion of the contact electrode melting or vaporizing due to the extreme heat of the arc. The extreme heat may also weld the contacts together, thereby making the relay unusable. In addition, these arcs may cause a build up of carbon deposit on the contacts, which, over time, accumulate to form a high resistance contact between the contacts, thus reducing the current flow to the load and making the relay less efficient.
Such arcs can generally, be suppressed by eliminating the voltage difference or current flow across relay contacts while switching the relay. This has been accomplished in the past by turning the load on with a triac while switching the relay on or off. Unfortunately, these triacs provide a path bypassing the high level of isolation offered by electromechanical relays. Moreover, triacs will also often fuse from the high inrush currents characteristic of certain loads.
In recent years some attempts have been made to control the physical opening and closing of an electromechanical relay at a point as close as possible to zero voltage in the sine waveform. For example, one technique is based on an assumption that zero voltage points correspond with zero current points. A complicating factor, however, is that in AC circuits, inductors and capacitors generally introduce phase shifts between voltage and current across a given component. Thus, in some instances, voltage zero cross is out of phase with current zero cross. In such instances, opening the relay at a zero voltage would not effectively prevent arcing.
Furthermore, other methods of determining current zero cross generally involve using an expensive current transformer with associated circuitry in order to dynamically measure the load current for a relay. The use of such current monitors, however, is generally both complicated and expensive.
These and other disadvantages and/or limitations are addressed and/or overcome by the assemblies, systems, and methods of the present disclosure.